This invention generally relates to methods reliably detecting a position of an armature of a high-speed solenoid and more particularly to detecting when the armature moves.
There are many applications in which it is desirable to employ high-speed solenoids, i.e., solenoids with a cycle period less than 30 milliseconds, to perform control functions. These so-called high-speed solenoids may be distinguished from low speed solenoids by consideration of their respective electrical and mechanical time constants.
High speed solenoids have mechanical time constants that are lower than their electrical time constants. In other words, the armature of a high speed solenoid moves so rapidly that, comparatively, the time during which motion occurs is only a small fraction of the total time needed to produce a coil steady-state operating current.
It is difficult to discern exactly when, in time, an armature of a high-speed solenoid moves. A typical solenoid driving circuit requires some finite time to develop sufficient current to move an armature. Typically the armature motion may begin at some indeterminate time after initiation of the current buildup. For example, motion of the armature may begin 1.2 milliseconds after application of a driving voltage or such motion may begin 1.7 milliseconds after the application of the voltage. In the context of low-speed solenoids, an error of position discernment of a millisecond or less is inconsequential. But in the context of high-speed solenoids, such an error in motion discernment substantially reduces the utility of the solenoid.
In the prior art, sensing circuits have been devised to detect motion of an armature of a solenoid. These prior art circuits detect discontinuities in coil-current change and correlate these discontinuities with changes in motion of an armature of a solenoid. One example of such a system is described in U.S. Pat. No. 5,995,356 (Glavmo, et. al.). In Glavmo, fuel injectors for an engine are operated in a synchronous relationship with a solenoid operated control valve. A detection circuit determines the times at which the valve reaches its open position and its closed position. The circuit detects discontinuities of current change in an operating coil of the solenoid. A unique discontinuity develops when the valve reaches its open position and a different discontinuity develops when the valve reaches its closed position.
Sensing circuits such as those described in Glavmo, require a very precise matching of coil inductance and timing of application of driving voltage. In other words, any particular solenoid must be matched with a particular timing of application of driving voltage. Only with proper matching will measurable current-change discontinuities occur when an armature of the solenoid reaches one of its end points. If driving voltage is applied too early or too late, discontinuities are smoothed out and become undetectable.
This is an acceptable constraint in solenoid valves for fuel injection systems such as those disclosed in Glavmo. These valves typically have duty cycles with periods in excess of 2000 to 3000 milliseconds. Such constraints are not acceptable in the operation of high-speed solenoids. It is impracticable to match driving voltage application timing with a particular inductance of a coil of a high speed solenoid. High speed solenoids operate with duty cycles having periods that are orders of magnitude less than the valves described in Glavmo, i.e. 30 milliseconds or less as compared to 2000 to 3000 milliseconds.
In another prior art sensing technique, separate sensors are employed along with high-speed solenoids to accurately determine when an armature moves. But, there are certain applications in which it is impracticable to employ a separate sensor to accurately discern armature motion. For example, use of separate sensors is undesirable in the context of space vehicles, because the sensors add weight to a vehicle and because they contribute to a reduced system reliability of the vehicle.
In some prior art applications, particularly high voltages are used to activate high speed solenoids. Higher voltages provide more rapid coil current rise to a steady state and consequently a tighter relationship between a time of application of a voltage and a time of movement of an armature. But, high voltage solenoids are larger and heavier than solenoids designed to operate with lower voltages. Increased weight in an aircraft or a space vehicle is an obvious disadvantage. Additionally, higher operating voltage may create undesirable corona effects.
An example of some difficulties which arise from these prior art shortcomings can be understood by considering the operation of turbine generators used in space vehicles. Solenoids are used to control intermittent bursts of oxygen and hydrogen flow into a combustion chamber of a turbine generator. Speed of the turbine generator is maintained at a desired rate by coordinating operation of solenoid valves to deliver proper amounts of these gases to produce a level of energy that corresponds to a varying electrical load on the turbine generator. This coordination must be particularity precise in order to assure that hydrogen is always present in a combustion chamber before oxygen is introduced to the chamber.
As electrical loads on the turbine generator vary, these bursts of gas flow must occur for varying lengths of time. In this context it is imperative to know, at any relevant time, whether a solenoid operated valve is in an open or closed state.
These turbine generators could not heretofore be operated at a single optimum speed. Changes in bursts of hydrogen and oxygen could not be made rapidly enough to compensate for variations in electrical load on the generator. Consequently, these turbine generators were operated with a relatively wide range of speeds. A low end of a speed range developed under high load conditions and a high end of the speed range developed under low load conditions.
Operating turbine generators with a wide speed range is inefficient. Such turbines consume an excess amount of fuel. By employing high-speed solenoid valves with improved correlations of action, it would be possible to attain a narrowing of a speed range of these turbine generators and thereby improve their efficiency. But this must be accompanied with an ability to determine the state of a valve at any given time. Such an improvement has heretofore required use of separate position sensors on the valves or high-voltage solenoids to actuate the valves. Use of separate position sensors or high-voltage solenoids are undesirable in the context of a space vehicle because they add weight to a vehicle and, because they contribute to a reduced reliability of the vehicle. Additionally, high voltages may create a corona hazard. In this setting it would is desirable to use current-change discontinuity measurement in a timing system to correlate operation of these valves.
As can be seen, there is a need for simple and accurate method and apparatus for discerning the state of an armature in a high-speed solenoid that is used as an actuator. Such a need for this method and apparatus also includes a need to avoid use of separate armature position sensors.